Okay, here are my general points of advice for running osu! in Arch Linux (I recently bought a new hard drive and had to do it all again)
1. Create a NEW, FRESH 32-bit wineprefix. ($ WINEARCH=win32 WINEPREFIX='~/.wine'; rm -rf ~/.wine; wine 'wineboot') or so
2. Install .NET 2.0 ($ winetricks dotnet20)
3. Install image libraries. (# pacman -S lib32-libpng lib32-giflib lib32-libjpeg-turbo)
4. Install graphics libraries. (# pacman -S lib32-mesa) but choose your graphics card of course ($ lspci | grep VGA)
4. Set proper audio output in Wine. ($ winecfg)
DO NOT run osu! with Mono though that seems counter-intuitive; it will immediately throw an InvalidProgram exception.
If you get pink sliders and backgrounds do not load, etc, you need the 32-bit image libraries mentioned above.
If you aren't using open-source driver for your graphics card, you probably should be, but it's your system so you know best.
These steps work for me. It is very finicky but once you get it, it should run absolutely fine. Good luck